Will changing the 3-prong plug from a dryer to range for high btu heater rated at 30-50A reduce possible output of 30A appliance?

It is not safe to change the 3-prong plug from a dryer to a range for a high-BTU heater rated at 30-50A. Doing so could cause the heater to draw more current than the circuit is designed for, which can lead to overheating, fire, or other safety hazards.

The heater's power cord should be properly rated for the appliance's maximum current draw. A dryer typically uses a 30-amp circuit, while a range uses a 50-amp circuit. The heater you have requires a cord that is rated for at least 50 amps.

If you are not sure what amperage your heater draws, you can check the heater's specifications or consult with an electrician.